5.5. Spectral Resolution

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Objectives

The purpose of this learning object is to introduce the concept of spectral resolution.

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Spectral resolution refers to the number and dimension of specific wavelength intervals of electromagnetic radiation that a sensor is capable of measuring.

The portions (ranges) of the spectrum to which an instrument is sensitive are referred to as its bands. A sensor can have multiple bands and bands can be of different widths. However, within each band, no variation can be resolved. The width of each band determines its spectral resolution. In this example, the resolution of band A is lower than that of band C which is in turn lower than that of band B:

A panchromatic image consists of one wide band that encompasses a large spectral range (often the entire visible spectrum).

A multispectral image consists of several relatively narrower bands of which two or three bands are in the visible range and there may also be some in near-infrared or middle infrared bands.

A hyperspectral image consists of a large number of narrower bands spread across the electromagnetic spectrum.

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Spectral Resolutions: Terms Explained

Panchromatic

Multispectral

Hyperspectral (Hyperion sensor bands)

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A practical example is provided by the Landsat Thematic Mapper (TM) sensor which is a multispectral sensor having seven bands, not all of which have the same spatial resolution.

Wavelength	Resolution (micrometers)	GRE (m)
Band 1	0.45 – 0.52	30
Band 2	0.52 – 0.6	30
Band 3	0.63 – 0.69	30
Band 4	0.76 – 0.9	30
Band 5	1.55 – 1.75	30
Band 6	10.4 – 12.5	120
Band 7	2.08 – 2.35	30

 

By contrast, the Medium Resolution Imaging Spectrometer (MERIS) (no longer in orbit) had 15 bands:

Band No.	Band centre (nm)	Width (nm)
1	412.5	10
2	442.5	10
3	490	10
4	510	10
5	560	10
6	620	10
7	665	10
8	681.25	7.5
9	708.75	10
10	753.75	7.5
11	760.625	3.75
12	778.75	15
13	865	20
14	890	10
15	900	10

 

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Reflection

	Spectral reflectance
	Blue (0.4 – 0.5)	Green (0.5 – 0.6)	Red (0.6 – 0.7)	Near Infrared (0.7 – 1.3)
Forest	29	36	27	56
Water	23	19	13	8
Corn	58	59	60	71
Pasture	39	42	32	62

 

The table shows the spectral reflectance characteristics of four land uses across four wave bands. Using these data, consider the following two questions:

(a) Plot spectral signature for Forest, Water, Corn and Pasture using the data from the table above. (X axis-Wavelength, Y axis Spectral reflectance)

(b) Which of the two band combinations are most useful for distinguishing between these classes? (Hints: use different band combination scatter plots)

Show Answer

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